Measuring film etching uniformity during a chemical etching process

ABSTRACT

A contactless method and apparatus for real-time in-situ monitoring of a chemical etching process during etching of at least one wafer in a wet chemical etchant bath are disclosed. The method comprises the steps of providing two conductive electrodes in the wet chemical bath, wherein the two electrodes are proximate to but not in contact with a wafer; monitoring an electrical characteristic between the two electrodes as a function of time in the etchant bath of the at least one wafer, wherein a prescribed change in the electrical characteristic is indicative of a prescribed condition of the etching process; detecting a minimum and maximum value of the electrical characteristic during etching; determining the times of the minimum and maximum values; and comparing the times of the minimum and maximum values to determine a film etching uniformity value. Such a method and the apparatus therefor are particularly useful in a wet chemical etch station, and are useful for film deposition process quality control.

The application is a division of application Ser. No. 08/269,861, filedJun. 30, 1994.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method and apparatus formonitoring the etching condition of a chemical etching process, and moreparticularly, to an improved contactless real-time in-situ method andapparatus for the same.

2. Discussion of the Related Art

Etching rates and etch end points must be carefully monitored andcontrolled in order to end etching processes at the desired time. Insemiconductor processing, inadequate or excess etching time can resultin undesirable film patterning. For instance, for semiconductor deviceshaving film layers or features in the micron and sub-micron range, aninadequate etch or an excess etch would result in the insufficientremoval or the excess removal of a desired layer. Insufficient removalof a desired layer can result in an undesired electrical open orelectrical short when the desired layer to be removed is an insulator ora conductor, respectively. Additionally, if the etch is in excess,undercutting or punch through can occur resulting in poorly defined filmpatterning or total lift-off. Inadequate or excess etching time furtherleads to undesirable reliability problems in the subsequently fabricatedsemiconductor device. As a semiconductor wafer is extremely expensivedue to many processing steps involved in the making thereof, the need tocritically control the etching end point in an etching process is highlydesirable.

An etch end point must be accurately predicted and/or detected toterminate etching abruptly. Etch rates, etch times, and etch end pointsare difficult to consistently predict due to lot-to-lot variations infilm thickness and constitution, as well as etch bath temperature, flow,and concentration variability. That is, an etch rate is dependent upon anumber of factors, which include, etchant concentration, etchanttemperature, film thickness, and the film characteristics. Precisecontrol of any of these factors can be very expensive to implement, forexample, concentration control.

Film etching nonuniformity is decidedly disadvantageous in semiconductorprocessing. Where there is a spatially distributed film non-uniformityin a film to be etched, wafers must be overetched to completely etch thelast-to-clear regions of the film. Thus there is necessarily a certainamount of overetching required. Non-uniformity can result fromdifferences across the wafer in film thickness, or can result fromdifferences in the physical or chemical properties of the film such asstoichiometry, density, or intrinsic stress. However, substantialoveretching can lead to wafer yield loss and the decreased reliabilityof the resulting electronic devices. In addition, circuit dimensionsmust be made larger to allow for overetch tolerances. Therefore, uniformfilms are highly desirable in the manufacture of semiconductor devices.The optimal development environment for designing processes andprocessing tools that produce uniform films would have a quick,inexpensive, facile and accurate means of measuring total filmuniformity. During the optimization of film deposition or growth, it ishighly desirable to have quick feedback on the influence of processvariables on the uniformity of the resulting film.

Currently, most etch rate end point determination techniques depend onindirect measurement and estimation techniques. Some etch monitoringtechniques have relied on external measurements of film thicknessfollowed by etch rate estimation and an extrapolated etch end pointprediction. However, etch rates may vary due to batch-to-batchdifferences in the chemical and physical characteristics of the film orthe etchant. These extrapolation methods are inadequate. Interruptedmeasurement techniques are also imprecise where the etch rate is notlinear, such as where an induction period occurs at the beginning of theetch.

Previous methods for measuring film etching uniformity include opticaltechniques such as ellipsometry, reflectance spectroscopy, and the prismcoupler method, on blanket films on monitor wafers. Film thicknessesmeasured on monitor wafers and even fiducial sites on product wafers donot always correlate to the actual film thicknesses in the region ofinterest (e.g. in contact holes, on stacks of films, etc.) in thedevice. These measurements are spatially discrete. They can betime-consuming especially when "complete" mapping of the thicknessnonuniformity is needed to determine the maximum and minimum pointsacross the film. Furthermore, optical measurements require expensiveequipment and specialized training for unambiguous interpretation ofresults. They usually assume refractive index dispersion relations andoptical constants of underlying films and substrates, which may beinvalid. In addition, these techniques have limitations in the thicknessranges for which they are applicable.

Other methods include similar optical measurements of fiducial regionsor discrete test wafers. However, such methods are expensive as portionsof the wafer are occupied by non-product fiducial areas or requireadditional test wafers. Such optical methods are also subject touncertainty resulting from turbidity of the etch bath and other opticaleffects and uncertainty resulting from non uniform films. Finally, suchoptical methods are subject to imprecision in the resulting estimate ofoveretch when the number of measured sites is insufficient or when thesections are not representative of the whole.

Real-time, in-situ monitoring is preferred. Some in-situ techniquesmonitor the etch rate of a reference thin film. This may requireadditional preparation of a monitor wafer containing the reference thinfilm or a suitable reference may be unavailable. Still other techniquesrequire physical contact of electrical leads with the wafer being etchedand electrical isolation of those leads and associated areas of thewafer from the etchant. This presents problems associated withcontamination, contact reliability and reproducibility, and the physicalconstraints which affect ease of use in manufacturing or automation. Yetother in-situ techniques monitor the etch rate of a fiducial region ofthe product wafer and require optical access to the wafer in the wetetch bath. Such methods are expensive as portions of the wafer areoccupied by non-product fiducial areas. Such optical methods are alsosubject to uncertainty resulting from turbidity of the etch bath andother optical effects and uncertainty resulting from non uniform films.

It would thus be desirable to provide an improved method and apparatuswhich provides non-contact, real-time, in-situ monitoring of an etchingcondition of a wafer being etched.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome the problems in theart discussed above.

Another object of the present invention is to provide an improvednon-contact method of monitoring the etching condition of a wafer beingetched.

Yet another object of the present invention is to provide an accuratereal-time, in-situ method and apparatus for monitoring an etchingcondition of a wafer being etched.

Yet another object of the present invention is to provide an accuratereal-time, in-situ method and apparatus for controlling a wafer etchingprocess.

According to the present invention, a contactless method for real-timein-situ monitoring of a chemical etching process for the etching of atleast one wafer in a wet chemical etchant bath comprises the steps of:

a) providing two conductive electrodes in the wet chemical bath, whereinsaid two electrodes are proximate to but not in contact with the atleast one wafer;

b) monitoring an electrical characteristic between the two electrodes asa function of time in the etchant bath of the at least one wafer,wherein a prescribed change in the electrical characteristic isindicative of a prescribed condition of the etching process;

c) detecting a minimum value of said electrical characteristic duringetching;

d) determining the time of the minimum value of said electricalcharacteristic;

e) detecting a maximum value of said electrical characteristic duringetching;

f) determining the time of the maximum value of said electricalcharacteristic; and

g) comparing time of said minimum value and the time of said maximumvalue and determining a film etching uniformity value therefrom.

In addition, according to the present invention, a contactless real-timein-situ chemical etch monitor for providing an indication of aprescribed condition of an etching process of at least one wafer to beetched in a wet chemical etchant bath comprises a means foraccomplishing each of the aforesaid process steps.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other teachings and advantages of the presentinvention will become more apparent upon a detailed description of thebest mode for carrying out the invention as rendered below. In thedescription to follow, reference will be made to the accompanyingdrawings, in which:

FIG. 1 shows a simplified block diagram of a contactless real-timein-situ etching condition monitor according to the present invention;and

FIG. 2 shows a graph of monitored electrical characteristics accordingto the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Copending U.S. patent application Ser. No. 07/985,413, filed Dec. 4,1992, now U.S. Pat. No. 5,338,390, entitled "Contactless Real-TimeIn-Situ Monitoring of a Chemical Etching Process," assigned to theassignee of the present invention (attorney docket FI9-92-152), thedisclosure of which is hereby incorporated by reference into the presentapplication, describes a related method and apparatus for thecontactless, real-time, in-situ monitoring of a chemical etching processduring etching of a wafer in a wet chemical etchant bath, wherein thetwo conductive electrodes are proximate to but not in contact with theat least one wafer, and further wherein said two electrodes arepositioned on opposite sides of the wafer. Six copending U.S. PatentApplications, filed on even date herewith, which are Ser. No.08/269,864, entitled "MINIMIZING OVERETCH DURING A CHEMICAL ETCHINGPROCESS;" Ser. No. 08/269,862, entitled "REAL TIME MEASUREMENT OF ETCHRATE DURING A CHEMICAL ETCHING PROCESS;" Ser. No. 08/269,863, entitled"CONTACTLESS REAL-TIME IN-SITU MONITORING OF A CHEMICAL ETCHINGPROCESS;" Ser. No. 08/269,860, entitled "METHOD AND APPARATUS FORCONTACTLESS REAL-TIME IN-SITU MONITORING OF A CHEMICAL ETCHING PROCESS;"Ser. No. 08/269,859, now U.S. Pat. No. 5,451,289, entitled "FIXTURE FORIN-SITU NON-CONTACT MONITORING OF WET CHEMICAL ETCHING WITH PASSIVEWAFER RESTRAINT;" and Ser. No. 08/269,865, now U.S. Pat. No. 5,445,705,"METHOD AND APPARATUS FOR CONTACTLESS REAL-TIME IN-SITU MONITORING OF ACHEMICAL ETCHING PROCESS;" and which are assigned to the assignee of thepresent invention, describe improvements to the method and apparatus forcontactless, real-time, in-situ monitoring of chemical etching disclosedin the Ser. No. 07/985,413 application. The disclosure of the aforesaidsix copending applications is also hereby incorporated by reference intothe present application.

Referring now to FIG. 1, there is shown an improved contactless,real-time, in-situ monitor for providing an indication of a prescribedcondition and determining an overetch value in an etching processaccording to the present invention. The monitor comprises at least twoconductive electrodes 12 positionable inside an etchant tank 14. Etchanttank 14 is of an appropriate size for receiving at least one wafer 16 tobe etched. The at least one wafer 16 comprises a semiconductor waferhaving at least one film layer thereon which is desired to be removed bya chemical etchant bath 18. While only one wafer is shown, more than onewafer may be placed in the etchant bath 18.

Electrodes 12 are connected to an electrical characteristic monitoringdevice 20 by electrical wires 22. Electrical characteristic monitoringdevice 20 can comprise, for example, an impedance analyzer 24 and a datarecording and analyzing device 26, such as a computer or a programmablecontroller. Monitoring of the prescribed etching characteristic iseffected by electrically sensing, in-situ, changes in an electricalcharacteristic of the wafer, such as, the impedance or an element orelements of impedance (e.g., admittance, capacitance, inductance,reactance and/or resistance), between the two electrodes 12. Forexample, the real and imaginary parts of the impedance as a function oftime may be measured. The output from analyzer 24 is an electricalsignal which is proportional to a condition of the film to be etched andis a function of time.

In FIG. 2 there is shown a graphical representation as a function oftime of a typical output signal from the electrical characteristicanalyzer such as an impedance analyzer 24 described above. Moreparticularly, FIG. 2, represents measured capacitance as a function ofetch time, as disclosed in copending application U.S. patent applicationSer. No. 07/985,413. Starting point 102 corresponds to the start of theetching process. Region 103 of the curve corresponds to thinning of thefilm during the etch process. Minimum point 104 corresponds to the pointin time at which a first penetration of the film by the action of theetchant occurs. It will be understood that such first penetration oretch through or "opening" relates to a small portion of the overall areaof the film. Region 106 of the curve corresponds to the period duringwhich an increasing proportion of the film is etched away, that is, asthe proportion of the etched area or "open" area becomes larger.Inflection point 107 has been found in this invention to correspond to apoint in time at which a last etch through occurs, that is, the lastfeature to open has been penetrated, but not fully cleared, and at whichtime a "foot" usually remains. By "foot" is meant a last portion ofmaterial to be removed which remains within the partially open featureuntil the nominal feature size has been etched; in cross section a footwill usually be observed to extend into the open feature from the lowerportion of the feature sidewall. Maximum point 108 corresponds to apoint in time at which all features are etched to nominal size in anessentially uniform film. Region 110 of the curve corresponds to theperiod during which more than the nominal or desired amount of materialto be removed by etching is being removed from the substrate, forexample, by an undercutting process.

The time period between minimum point 104 and inflection point 107 inthe curve of FIG. 2 represents the extent of nonuniformity in the filmthickness for the case where the film is homogeneous, that is, has alinear etch rate per unit of thickness at all points on the wafer. Theshorter the time duration, the more uniform the film thickness is. Thecontrary also holds true. Based on the time duration between thesepoints, the extent of uniformity of the film can be immediatelydiscerned. Alternatively, where thickness can be shown to be extremelyuniform, the nonuniformity may be attributed to other physical orchemical phenomena. This information is not from a series of discretemeasurements across the film, as in optical approaches, but is from asingle measurement representative of the whole wafer. In onemeasurement, the relative difference between the absolute maximum andabsolute minimum thickness is obtained. In deposition processdevelopment, the goal is to maximize the corresponding slope on thecharacteristic curve obtained by the wet etch monitor system.

Returning to FIG. 1, the data recording and analyzing device 26 receivesthe output signal from the analyzer 24. Data recording and analyzingdevice 26 comprises well known elements of (a) means 40 for detecting aminimum value of the electrical signal as a function of time; (b) means44 for detecting a maximum value of the electrical signal as a functionof time; (c) means 42 for determining the time of the minimum value; (d)means 46 for determining the time of the maximum value; (e) means 48 forcomparing time of said minimum value and the time of said maximum valueand determining a film etching uniformity value therefrom. It will beapparent that the means for detecting either a maximum or a minimum orboth may be combined with the means for determining the respective timeor times thereof.

As further shown by FIG. 1, an alternative embodiment of the presentinvention may include a means 60 for determining an etching end point,which means is responsive to any one or more of the time of the minimumvalue, the time of the maximum value, the time of the inflection point107, and the etching uniformity value. Means 60 for determining anetching end point may be separate from, or part of, electricalcharacteristic monitoring device 20.

In yet another alternative embodiment, the apparatus of the presentinvention may comprise a means 70 such as a computer or a programmablecontroller which is responsive to the end point determined by means 60,and which means 70 may, in turn, control the etch process such as byactuating a wafer handling means 80. Furthermore, electricalcharacteristic monitoring device 20 can likewise comprise an impedanceanalyzer and a computer or a programmable controller, the computer orprogrammable controller providing feedback control to initiate, control,and terminate an etching operation. Impedance analyzers, computers, andprogrammable controllers are well known in the art.

Not shown, but contemplated as an alternative embodiment within thescope of the present invention, is a method and apparatus which detectsand is responsive to inflection point 107 in determining a film etchinguniformity value and/or end point. For example, the practitioner willunderstand that the maximum point may be anticipated from the detectedminimum point and the inflection point, and the film etching uniformitydetermined therefrom in order to minimize overshooting the etching endpoint.

Finally, it is noted that many of the disclosed means may be assembledas discrete elements or together in a combined element without affectingthe essential function thereof.

It should be understood that the invention contemplates that the shapeof the curve shown in FIG. 2 may deviate from that shown in the figure,provided however, that a maximum point and a minimum point must bedetected, or alternatively, at least one of a maximum point or a minimumpoint must be detected together with inflection point 107. The order ofoccurrence of minimum point 104 and maximum point 108 of the curve maybe inverted, with the maximum occurring before the minimum. Furthermore,the in situ etch monitor may record the etch rate as a function of time.This results in an etch record which is essentially amonolayer-by-monolayer etch rate depth profile of the thin film strata.Any change in the ongoing etch rate is observed as a change in the curveshape. Thus, any significant batch-to-batch variation in the homogeneityof a film that would affect the etch rate will be reflected in thebatch-to-batch reproducibility of the etch records. For example, atemporary or intermittent pressure or electrical fluctuation in a filmdeposition process could result in transient density or stoichiometrychanges in the resulting film. Such changes would appear as small spikesor plateaus in the etch record. Batch-to-batch thin film thickness anduniformity reproducibility may be correlated and monitored bycorrelating and monitoring etch monitor results.

Thus there has been shown an improved real-time in-situ monitoringmethod and apparatus which provide accurate, non-contact, monitoring ofan etching characteristic of an etching process. Such a method andapparatus are inexpensive to implement and ensure the integrity of theetched wafer or wafers. Etching of a wafer can be controlled precisely.

While the invention has been particularly shown and described withreference to specific embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade thereto, and that other embodiments of the present invention beyondembodiments specifically described herein may be made or practicedwithout departing from the spirit of the invention. System conditionparameters, such as impedance analyzer frequency, etc., may need to beadjusted accordingly to obtain optimum detection sensitivity. Similarly,other changes, combinations and modifications of the presently disclosedembodiments will also become apparent. The embodiments disclosed and thedetails thereof are intended to teach the practice of the invention andare intended to be illustrative and not limiting. Accordingly, suchapparent but undisclosed embodiments, changes, combinations, andmodifications are considered to be within the spirit and scope of thepresent invention as limited solely by the appended claims.

What is claimed is:
 1. A method for contactless, real-time, in-situmonitoring of a chemical etching process during etching of at least onewafer in a wet chemical etchant bath, said method comprising the stepsof:a) providing two conductive electrodes in the wet chemical bath,wherein said two electrodes are proximate to but not in contact with theat least one wafer; b) monitoring an electrical characteristic betweenthe two electrodes as a function of time in the etchant bath of the atleast one wafer, wherein a change in the electrical characteristic isindicative of a state of the etching process; c) detecting a minimumvalue of said electrical characteristic during etching; d) determining atime of the minimum value of said electrical characteristic; e)detecting a maximum value of said electrical characteristic duringetching; f) determining a time of the maximum value of said electricalcharacteristic; and g) comparing the time of said minimum value and thetime of said maximum value and determining a film etching uniformityvalue therefrom.
 2. The method of claim 1, wherein the monitoring of anelectrical characteristic in step (b) comprises monitoring impedance andfurther wherein the change in the electrical characteristic comprises achange in impedance.
 3. The method of claim 2, wherein the monitoring ofan electrical characteristic in step (b) comprises monitoring impedanceand further wherein the change in the electrical characteristiccomprises a change in a component of impedance, wherein said componentis selected from the group consisting of admittance, reactance,resistance, capacitance, and inductance.
 4. The method of claim 1,wherein an etching end point is determined in real time in response tosaid film etching uniformity value and an etching starting point.
 5. Themethod of claim 4, further comprising the step of controlling theetching process in response to the etching end point, wherein the endpoint is determined in real time.